The research plan proposes the cloning, characterization of expression and initial functional analysis of a novel extracellular matrix protein in the developing rat heart. The gene encoding this protein (gene X) initially was identified in the human fetal adrenal by the candidate while working in the sponsors laboratory. Happily, recent observations have shown that there is significant gene X expression in human fetal heart. Protein homology analysis has demonstrated significant homology between the gene X protein and well-characterized extracellular matrix proteins, including fibronectin and tenascin, both of which appear to be important in cardiac morphogenesis. Progress on the human gene product has been slowed by the lack of availability of high quality human tissues for analysis. There is strong presumptive evidence that gene X exists and is expressed in fetal rat heart. A more complete understanding of the function of the gene X protein mandates development of a model animal system, a goal which this proposal will accomplish.